To this end, the present invention provides a device for operating an electric machine comprising an inverter, which is designed to electrically separate the connections of the electric machine from one another in an idling mode and to electrically connect the connections of the electric machine to one another in a short circuit mode; wherein the inverter is furthermore designed to then switch from the idling mode to the short circuit mode if an electrical voltage has a predetermined value at the connections of the electric machine.
The present invention further provides a method for operating an electric machine, comprising the following steps: operating the electric machine in an idling mode, determining an electrical voltage at the connections of the electric machine, and short circuiting the connections of the electric machine if the electrical voltage has a predetermined value at said connections of the electric machine.
The concept underlying the present invention is to limit the switch from an idling mode to an active short circuit mode of an electric machine to precisely defined parameters. A switch from the idling mode to the active short circuit mode results only if defined voltage conditions are present at the connections of the electric machine. In this way, transient phase currents occurring when switching from the idling mode to the active short circuit mode can be reduced. This reduction of the phase currents has a positive effect on the service life of the components in the actuating circuit for the electric machine. In addition, the components used in said circuit can be of lighter construction and therefore more cost effective due to the expected phase currents being reduced. The reduced transient phase currents also have a positive effect on the sizing and the service life of the electric machine itself.
The voltage values and thus the parameters at which the electric machine is to be switched from the idling mode to the active short circuit mode can be determined in advance. As a result, the inventive switch from the idling mode to the active short circuit mode can be very easily carried out without inducing disproportionately large phase currents in the process.
According to one exemplary embodiment, the device further comprises a voltage sensor, which is designed to detect the electric voltage at the connections in the electric machine. The switch from the idling mode to the short circuit mode then occurs if the electrical voltage, which was detected in this manner, at the connections of the electric machine has a predetermined value. As a result, the switch from the idling mode to the active short circuit mode can be especially easily initiated.
According to one exemplary embodiment, the device further comprises a rotary encoder, which is designed to detect an angular position of the rotor of the electric machine, wherein the electrical voltage at the connections of the electric machine is determined on the basis of the angular position of the rotor of the electric machine. By means of the rotary encoder, an inference can be made about the position of the rotor and therefore about the voltage conditions at the connections of the electric machine in the idling mode. Hence, a direct determination of the voltage conditions at the electrical connections is not necessary. Nevertheless, an efficient and gentle switch from the idling mode to the short circuit mode can be implemented in this manner.
According to one exemplary embodiment, the inverter is designed to then switch from the idling mode to the short circuit mode if the amount of voltage is at a maximum in one strand. If an AC voltage is applied to the strands of the electric machine, a particularly gentle switch from the idling mode to the active short circuit mode can take place especially if one of these AC voltages has just reached a maximum value.
According to one exemplary embodiment, The inverter comprises a plurality of semiconductor switches, the electrical connections being electrically connected to one another via the semiconductor switches in the short circuit mode. Furthermore, the semiconductor switches preferably each comprise a freewheeling diode that is connected in parallel. Such semiconductor switches are particularly well suited for actuating an electric machine and also for actuating the electric machine in the freewheeling mode as well as in the active short circuit mode.
According to one exemplary embodiment, the present invention relates to an electric drive device comprising an electric machine and a device according to the invention for operating an electric machine.
According to one exemplary embodiment, the electric machine comprises a permanently excited synchronous machine.
According to a further exemplary embodiment, the present invention comprises a motor vehicle having an electric drive system, which comprises a device according to the invention for operating an electric machine. According to a further exemplary embodiment, the method for operating an electric machine further comprises a step for detecting a request for short circuiting the connections of the electric machine, wherein, upon receiving the request, the step for actuating the electric machine delays the short circuiting until the voltage at the connections of the electric machine has the predetermined value.